~50C looks about right. Not much oc you can do with the stock HSF. My E6400 used to run with an Arctic Cooling 7Pro. Very little good cooler - the CPU was oc'ed to 3.43, and the load temp while folding were ~55C (test bench setup).

Coretemp seems to be the best temp indication of all of them, but there is an issue with how it reads freq (I think it just does a FSB X normal multiplier which is why we can end up with higher than actual freq readings).

Something else to keep in mind is that there can be issues with the CPU sensors themselves... I just had to swap the CPU between my bench rig and my main rig because the bench rig was reading too low.